1. Field of the Invention
The present invention relates in general to a method and apparatus for placing a pipe string in a well and, in particular, to an apparatus and method for gripping a pipe string during placement within a well.
2. Brief Description of Related Art
Drilling operations use gripping assemblies to manipulate pipe strings into and out of a well. Gripping assemblies generally consist of pipe slips and the associated equipment needed to operate the pipe slips. This equipment is configured to variably engage and hold a pipe string passing through an opening defined by the pipe slips. Generally, the pipe slips have a surface approximately parallel to an axis of the pipe string being manipulated. These pipe slip surfaces are moved into and out of contact with the surface of the pipe string to alternately grip and release the pipe. The gripping assembly grips and holds the pipe string while the pipe string is moved or while additional pipe or objects are attached to or removed from the pipe string.
Some gripping assemblies require multiple movable gripping assemblies to effectively grip and hold a pipe string while additional pipe is added to or removed from the pipe string. In these systems, there is a complicated choreography of movement between at least two movable gripping assemblies. A first holds the pipe string while a second brings a new pipe element to be coupled to the pipe string. Following coupling, the second holds the pipe string via the new pipe element while the first releases the pipe string and is moved to procure another pipe element. The first then brings the pipe element over to the pipe string where the process repeats. In practice, this is a complicated process with many moving parts that can damage the pipe string, pipe elements, and potentially the workers orchestrating the process. In addition, the constant changing between gripping assemblies often misaligns the pipe string relative to the wellbore. Therefore, there is a need for a gripping assembly that uses only one movable gripping assembly.
Many times, the pipe slip surfaces have a series of protrusions, i.e. pipe slip teeth, on the surface adjacent to the pipe string. These pipe slip teeth are moved into contact with the pipe string surface, and the pipe string is then lowered to engage the pipe slip teeth into the surface of the pipe. In this manner, the pipe string is held in place by the pipe slip teeth. Unfortunately, the pipe slip teeth may mar and damage the pipe string at the point of contact between the pipe slip teeth and the pipe string. After additional pipe is added to the pipe string, the gripped portion of the pipe string is then lowered further into the well; thus, the damaged portion of the pipe string becomes exposed to the harsh environment of the well. The damaged portion then becomes a point of failure as the pipe string is more likely to corrode and fail at the location where the pipe slip teeth gripped the pipe string. This problem is exacerbated if the pipe in the pipe string is specially coated because the pipe slip teeth may dig into and remove portions of the coating at the point of contact. Thus, there is a need for a gripping assembly that can grip a pipe string without the use of pipe slip teeth.
Some attempts to overcome the problems caused by pipe slip teeth use smooth faced surface pipe slips at the point of contact with the pipe string. The smooth faced surface pipe slips are coupled to complex systems designed to exert a substantial radial force on the surface of the pipe string. Due to the complexity of such systems, they are difficult to produce and maintain. In addition, the complexity significantly increases the costs of the equipment needed to drill a well and turn the well to production. Furthermore, current smooth faced surface pipe slip systems must use a friction paper interposed between the pipe string and the pipe slips. The friction paper helps set the pipe string within the pipe slips. However, the friction paper also binds and can interfere with the gripping assembly. In some cases, friction paper binding can deform the surface of the pipe string. Therefore, there is a need for a smooth faced pipe slip system for gripping pipe strings that makes use of simpler means to exert a sufficient radial force on the pipe string without the use of friction paper.
Other attempts to overcome the problem pair simpler smooth faced surface pipe slip systems with a backup pipe slip system using pipe slip teeth. These systems first pass the pipe string through a primary smooth faced surface pipe slip system and then through a secondary pipe slip system using pipe slip teeth. Because the primary system is backed up, the associated equipment used to exert a radial force on the pipe slips does not need to be as robust. When the primary system fails, the secondary system will then engage and prevent the pipe string from slipping further into the well. While these devices overcome the complexity problems of other smooth faced surface pipe slip systems, many times they are unable to exert the necessary radial force against the pipe string leading to frequent failure. When the primary system fails to adequately grip and hold the pipe string and the pipe string begins to slip, the movement of the pipe string will force the pipe slip teeth of the backup system into the pipe string, preventing the pipe string from falling too far and doing too much damage. However, some damage will occur to the pipe string before the backup pipe slip teeth halt and re-grip the pipe string. This also may cause the pipe string to be misaligned in the well. In addition, the lack of necessary radial force limits the weight of the pipe string that the gripping assembly can support. Therefore, there is a need for a smooth faced surface pipe slip system that does not use a backup system with pipe slip teeth.